1. Field of the Invention
The present general inventive concept relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus to allow a variety of discs having different recording densities to be compatible with each other.
2. Description of the Related Art
Generally, a conventional apparatus for recording/reproducing data in/from an optical disc (hereinafter referred to as a conventional optical-disc recording/reproducing apparatus), acting as an information storage medium using a light beam (or optical signal) focused by an objective lens, determines its own recording capacity according to a size of a beam spot (S). The size of the beam spot (S) is determined by the following equation 1 according to a wavelength of the light beam λ and a Numerical Aperture (NA) of the objective lens:S∝λ/NA  [Equation 1]
Therefore, in order to reduce the size of the beam spot focused on the optical disc to implement a high-density optical disc, a short-wavelength light source, such as, a blue-violet laser beam, and an objective lens having an NA of at least 0.6 are needed in the optical-disc recording/reproducing apparatus.
A Digital Versatile Disc (DVD) performs recording/reproducing of data using an optical signal having a wavelength of 650 nm (or 635 nm) and an objective lens having an NA of 0.6. If the DVD has a diameter of 120 mm and a track-pitch of 0.74 μm, a cross-section of the DVD has recording capacity of at least 4.7 GB.
Therefore, the above-mentioned DVD is insufficient as a recording medium capable of recording High Definition-grade (HD-grade) moving image data (or HD-grade motion picture data). In order to record moving image data of about 135 minutes at the HD grade, a cross section of the DVD requires a recording capacity of at least 23 GB.
In order to satisfy the increasing demands of the above-mentioned high-density recording capacity, an optical signal (i.e., a blue optical signal) having a wavelength (e.g., 405 nm˜408 nm) shorter than that of a red optical signal and an objective lens having an NA of more than 0.6 have been widely used. Many developers are conducting intensive research into a high-density optical disc having a narrower track (i.e., the next-generation DVD such as a HD-DVD), and the specification of the next-generation DVD is being intensively developed.
In the meantime, in order to guarantee a tolerance caused by a tilting of the optical disc, the thickness of the optical disc must be reduced in inverse proportion to the NA of the objective lens, resulting in the implementation of high-density optical discs. Considering the allowance tolerance caused by the tilting of the optical disc, the DVD has a thickness of 0.6 mm whereas the CD has a thickness of 1.2 mm, and the HD-DVD may have a thickness of 0.6 mm. In the case of the NA of the objective lens, the CD has a NA of 0.45, and each of the DVD and the HD-DVD may have a NA of 0.65. Considering the recording density of the light source of the HD-DVD, the HD-DVD may use a blue optical signal. In this way, an important matter in the development of the above-mentioned new-standard optical disc is the compatibility between the new-standard optical disc, such as the HD-DVD, and a conventional optical disc, such as, the DVD.
Blu-ray Discs (BDs) have also been widely developed as a next-generation high-density optical storage medium using an objective lens having a high NA (e.g., 0.85).
The BD uses a short-wavelength optical signal (i.e., an optical signal of 405 nm-408 nm) as a light source, and may have a thickness of 0.1 mm. The above-mentioned BD has a recording density ten times higher than that of the DVD. In this way, another important matter in the development of the new-standard optical storage medium is the compatibility between a new-standard optical storage medium, such as a BD, and the conventional discs, such as a DVD.
For example, a reflection factor of a write-once optical disc (e.g., DVD-R and CD-R) from among conventional optical discs is considerably decreased according to wavelengths, such that a light beam having a predetermined wavelength of 650 nm and a light beam having a predetermined wavelength of 780 nm are required to solve the problem of reflection-factor deterioration. Therefore, considering a compatibility of the DVD-R and/or the CD-R, each of a HD-DVD optical pickup unit and a BD optical pickup unit is required to use two or three light sources having different wavelengths.
However, high-grade complex techniques are required to design or manufacture an objective lens having a high NA (e.g., 0.85) in the form of a single piece, and it is difficult to manufacture an objective lens which is simultaneously suitable for the HD-DVD and having a high NA, that can also be applied to DVDs and/or CDs, and in addition has a long working distance as in the objective lens for DVDs. Also, the specifications of the new-standard high-density discs (e.g., HD-DVDs and BDs), each of which has a short-wavelength of 405 nm, have not been standardized yet. In order to manufacture an optical pickup unit capable of coping with two discs simultaneously and being compatible with CDs and/or DVDs, a large number of optical components and control units are required, such that the configuration of the optical pickup unit becomes more complicated.